1. Field of the Invention
This invention relates to an optical pickup head for reading out signals from an optical disk for example.
2. Description of the Prior Art
There are various optical heads for optically reading information signals recorded to an optical disk such as a so-called digital audio disk. To meet the recent demand of miniaturizing a device and reducing the cost, the applicant has disclosed an optical head wherein a semiconductor laser device is used as a light source and a beam splitter prism and the semiconductor laser are provided on a semiconductor substrate with light receiving devices being formed on its surface in our PCT application Publication No. WO 87/05142.
The preceding optical head, the structure of which is illustrated in FIG. 1, consists of a semiconductor substrate 31 with light receiving devices 32 being formed on the surface thereof, a beam splitter prism 33 having almost trapezoidal sections fixedly mounted on the light receiving devices 32, and a semiconductor laser 36 provided on the semiconductor substrate 31 in front of a semitransmissive reflective film 34 provided on the inclined end surface of the beam splitter prism 33. In this optical head, a laser light irradiated from the semiconductor laser 36 is almost divided into two, namely a reflected light and a transmitted light at the semitransmissive reflective film 34, whereby the reflected light is ultimately collected by an objective lens 39 to be projected on a record surface of a disk 40 such as compact disk, while the transmitted light is refracted at a boundary surface of the prism 33 to be entered into the light receiving device 32. The reflected light is useful being used to read out information signals from the disk 40, whereas the transmitted light introduced directly from the semiconductor laser 36 to the prism 33 is a so-called stray light, having no necessity for optical signal detection.
The laser beam radiated on the disk 40 and reflected on the record surface thereof is entered transmitting the film 34 of the prism 33 through the objective lens 39 to form a so-called return light, which is detected by the light receiving devices 32. However, in contrast to the return light, since the above-mentioned unnecessary light being entered directly has the intensity or amount of light several times larger than that of the return light, it causes a DC (direct current) offset in the detected signals from the light receiving devices 32. This exerts a harmful influence on error detection signals in the light spots concentrated on the disk 40. Outputs due to the unnecessary light can be eliminated by adding a so-called DC offset eliminating circuit in a detection circuit as shown in FIG. 2. With reference to FIG. 2, the detected outputs offsetting said DC offset can be obtained from the output terminal 45 by supplying light detected outputs from a photodiode 42 corresponding to the light receiving devices 32 to an inverting input terminal of an operational amplifier 43 and by supplying outputs from a direct current voltage generating circuit 44 generating the direct current voltage V.sub.DC in accordance with said DC offset to a non-inverting input terminal of the operational amplifier 43. However, such DC offset eliminating circuit is required for every detecting device in the light sensor, so that the cost runs up due to an increase of numbers of circuit parts and of adjustment steps.
Besides, when the DC offset is caused in the optical detecting outputs as earlier noted, the optical output of the semiconductor laser and the sensitivity of the detector are fluctuated by temperature for example, and are to be sensed of the error detection signals as an error component, resulting in a mistaken control.
In order to avoid the foregoing problem or defect, the applicant has provided a semiconductor laser in our preceding U.S. patent application Ser. No. 41,140 now U.S. Pat. No. 4,772,784. In this application, there is disclosed a semiconductor laser device including a coating layer having more than two layers the refractive index of which is set such that the directly entered transmitted light or the stray light from the semiconductor laser with a greater incident angle than the prescribed one is almost totally reflected on the boundary surface of the semiconductor substrate and the beam splitter prism.
According to the technology of the invention, the optical detecting section is effectively irradiated only by the return light from the medium, and tracking or focusing control of the optical head and so on improves. Yet in this technology, it is difficult to control the film thickness of the coating layer. Since a manufacturing error of the film thickness becomes an error of the angle of separation of said stray light and detecting light, and the amount of light adjacent to the angle of separation changes slowly, it becomes further difficult to set the accurate angle of separation. Also, the additional coating film increases the cost.